1. Field of the Invention
This invention relates to rolls for removing air from spaces in machinery which processes traveling webs. More specifically, this invention relates to a roll for withdrawing air from the spaces between rotating rolls in a papermaking machine wherein air which is moving with the traveling paper web or felt, or both, tends to cause disruptions in the unsupported span of tee paper web between locations of its support on rotating rolls or other machine components. Still more specifically, this invention relates to a so-called felt turning roll for use in the dryer section of a papermaking machine wherein the roll has no central core shaft and is specifically adapted to provide support and guidance for the narrow paper web strip which is passed through the machine initially during the start-up procedure.
2. Description of the Prior Art
In prior felt turning rolls, such as is currently used in papermaking machinery, a stationary center shaft extends for the length of the roll about which a perforated roll shell is rotatably mounted. The center shaft is hollow, or bored, to permit a source of sub-atmospheric air pressure to be connected with one end of the shaft. The shaft contains openings to permit the interior space of the roll shell to be exposed to the source of sub-atmospheric air pressure whereby air is drawn into the roll through the holes forming perforations in the roll shell and which are drilled through the roll shell in a uniform pattern. This permits the roll to draw air from the space surrounding the roll, commonly referred to as a pocket in a papermaking machine, to interrupt the movement and diminish the amount of air traveling with the dryer felt, thereby mitigating potential separation of the web from the felt.
Such a structure operates satisfactorily, but it has limitations and shortcomings. First, the roll usually contains two or more longitudinally extending seals which define one or more vacuum chambers which can be oriented as desired. This causes uneven seal wear due to thermal distortion and deflection. Secondly, considering the relative long length of all rolls in modern papermaking machines, which may extend for greater than 8 meters along the face length of the rolls, and considering the relatively small diameter typical of such felt turning rolls, which diameter may typically be about 60 cm, the rolls can vibrate at certain machine speeds due to the relatively low natural frequency of the center shaft. Of course, any such vibration in a papermaking machine is deleterious to the papermaking process.